Often, in these cases, what technically skilled people fail to do is make the case in sufficiently precise dollars-and-cents terms (often, in part because they fail to realize how much business types--which Product Managers mostly are, in practice--respond to that kind of presentation, and also often because they are professionally inclined to not allow precision of their forecasts to exceed accuracy, whereas business types are often conditioned to both create and act on SWAGs with precise cost benefit estimates that, while they often have a post hoc rationalization, are essentially pulled out of the air.)

I'm not saying this is the issue in your case, but I've seen it a lot.